1. Field of the Invention
The present invention relates to electrode kinetics in electrochemical systems and, in particular, to techniques for improving electrode electrocatalysis in such systems. More particularly, the invention relates to the application of an electrochemical impedance spectroscopic method for determining the Langmuir or the Frumkin adsorption isotherms.
2. Background of the Related Art
Adsorption of a reactive species on an electrode surface has important consequences in an electrochemical system. Adsorption may hinder the electrode reaction by forming a blocking layer on the electrode surface; alternatively, adsorption may enhance reactivity of an electrochemical system by increasing the reactivity of the adsorbed species through dissociation of a nonreactive material into reactive species. In the latter situation, the electrode functions as a catalyst for a reduction or oxidation reaction, a process termed xe2x80x9celectrocatalysis.xe2x80x9d The relationship among the amount of a particular species adsorbed on an electrode, the activity of the species in bulk solution, and the electrical state of the electrochemical system at a particular temperature is given by the adsorption isotherm. The relationship among the fractional electrode surface coverage of these species and the electrode kinetic parameters is described by the Langmuir and Frumkin adsorption isotherms. From the shape of the adsorption isotherms, the adsorption behavior can be interpreted. By examining the adsorption behavior of a reactive species, the optimal operating conditions for an electrochemical cell (e.g, operating potential, electrolyte concentration, electrode material/electrolyte composition combinations etc.) can be determined.
In the adsorption of ions on a dissimilar electrode material, the adsorbed species will be either more weakly adsorbed to the substrate than to a material of the same type or the adsorbed species will be more strongly adsorbed to the substrate than to a material of the same type. Species more strongly adsorbed to the dissimilar substrate are said to be underpotentially deposited, that is, they are deposited at potentials which are more positive than the equilibrium potential for deposition on a material of the same type. In underpotential deposition, the adsorbed species attempts to form a monolayer on the electrode surface.
Commercially important electrochemical systems frequently involve noble metal electrodes in acidic or alkaline aqueous solutions. In such systems, reduction of hydrogen ions at the cathode yields H2. The transition between the underpotentially deposited hydrogen and the overpotentially deposited hydrogen is important to understand the mechanisms of the H2 evolution reaction at the cathode.
Voltammetric and spectroscopic techniques have been used to study the adsorption processes of the underpotentially deposited hydrogen and the overpotentially deposited hydrogen on noble metal surfaces. In general, the underpotentially deposited hydrogen and the overpotentially deposited hydrogen occupy different surface adsorption sites and act as two distinguishable electroadsorbed hydrogen species. However, in the past the relation, transition, and criterion between the underpotentially deposited hydrogen and the overpotentially deposited hydrogen at the noble metal/aqueous electrolyte interfaces have been studied from the point of view of the hydrogen evolution reaction rather than focusing on the hydrogen adsorption sites and processes.
Although the Langmuir adsorption isotherm is regarded as a classical law in physical electrochemistry, it is useful and important for interpreting the relation, transition, and criterion between the underpotentially deposited hydrogen and the overpotentially deposited hydrogen and the two distinct adsorption sites for the cathodic H2 evolution reaction at the interfaces.
Thus, there is a need in the art for a fast, simple, and reliable technique to characterize the relation of and the transition between underpotentially deposited hydrogen and overpotentially deposited hydrogen at electrode hydrogen adsorption sites in electrochemical systems. Knowledge of this transition region would facilitate selection of the optimal operating conditions for an electrochemical system, in particular, the enhancement of electrocatalysis at an electrode, yielding electrochemical systems of maximum performance.
By the present invention, measurement techniques yield the Langmuir adsorption isotherms from which the transition region between underpotentially deposited hydrogen and overpotentially deposited hydrogen at an electrochemical system electrode is ascertained. According to the present invention, an electrochemical impedance spectroscopic method creates a phase-shift profile for the intermediate frequencies which can be used to determine the Langmuir or the Frumkin adsorption isotherm. The phase-shift method is convenient and useful for studying the electrode kinetics related to the Langmuir adsorption isotherm, the relation and transition between the underpotentially deposited hydrogen and the overpotentially deposited hydrogen, and the two distinct adsorption sites for the cathodic hydrogen evolution reaction at electrode/electrolyte interfaces. This method demonstrates that the criterion of the underpotentially deposited hydrogen and the overpotentially deposited hydrogen is the hydrogen adsorption sites and processes rather than the hydrogen evolution reaction, facilitating the study of intermediate adsorption processes in electrode kinetics. The technique of the present invention permits selection of the desired operating parameters for an electrode/electrolyte system thereby increasing electrode electrocatalysis and creating more efficient electrochemical systems.